I. Field of the Invention
This invention relates to cements used for bonding carbon blocks. More particularly, the invention relates to cements suitable for bonding carbon blocks used to form the cathodes of electrolytic reduction cells, as well as carbon blocks used for similar high temperature applications.
II. Description of the Prior Art
Aluminum is conventionally produced by the reduction of alumina in a "Hall-Heroult" electrolytic cell provided with a lining made of prebaked carbon blocks. The lining acts as a refractory material to protect the cell walls and bottom from the hot molten electrolyte and aluminium, and as a cathode for the electrolysis process. To form the cell lining, the prebaked carbon blocks are fitted together as closely as possible and the inevitable gaps present at the block joints are filled with a material which carbonizes at the operational temperatures of the cell so that a monolithic carbon lining is formed.
Any cracks which form in the carbon lining reduce the operational lifetime of the cell since the hot electrolyte or aluminum may then penetrate the protective lining. It is consequently important to use a material in the cathode block joints which has good resistance to cracking and shrinkage.
Hot tamping mixes have conventionally been used for filling cathode block joints. Such mixes normally consist of a carbonaceous aggregate, e.g. calcined anthracite, and a binder, e.g. pitch or a tar-pitch mixture. Hot tamping mixes achieve good results but they expose workers to unpleasant tar fumes and to noise generated by tamping tools. To overcome the problem of fume generation, various formulations which can be used at room temperature to fill cathode block joints have been developed. These formulations are of two distinctly different types, namely room temperature tamping mixes and room temperature cements (sometimes also referred to as "glues"). Room temperature tamping mixes are usually formed by adding a viscosity-reducing solvent to hot tamping mix formulations, but their room temperature viscosities remain fairly high (e.g. about 5 poise) and so tamping tools are required to pack the mixes into previously-formed cathode block joints. Consequently, the workers are still exposed to the noise of tamping tools when such formulations are used. In contrast, room temperature cements, which generally consist of a carbonaceous aggregate and a resin-based binder, have fairly low room temperature viscosities and may be spread with trowels or the like onto the cathode blocks before the joints are formed. Clearly, therefore, cements are preferred from the environmental point of view.
However, the inventors of the present invention have found that commercially-available room temperature cements are not satisfactory because they shrink and crack unacceptably when used in electrolytic cells. This may be because the commercial mixes were developed for joint thicknesses of about 1 mm, which are not unusual when graphite blocks are employed, whereas joint thicknesses of up to 3 mm are more usual when amorphous carbon blocks are employed because the machining costs of such blocks increase unacceptably when strict tolerances are imposed. The use of amorphous carbon blocks is common in electrolytic cells used for the production of aluminum, so there is a need for a room temperature cement which can fill joints up to about 3 mm (e.g. 2-3 mm) in width without cracking when exposed to cell-operating conditions.
It is believed that previous attempts to produce suitable cements concentrated on maximizing the density of the cured product to give a joint of high strength and low porosity, but such mixes result in high shrinkage. The inventors have found that cracking takes place if the linear shrinkage of the cement exceeds about 5% when the green cement is first subjected to cell-operating temperatures (e.g. about 900.degree.-1000.degree. C.) which cause the binder to carbonize. A small amount of shrinkage is permissible, indeed desirable, because adjacent carbon blocks expand when heated and so reduce the joint width, but linear shrinkages of more than about 5% exceed the reduction of the joint width and introduce the potential for lining failure. However, the inventors also found that it was not an easy matter to produce a room temperature cement having a linear shrinkage of less than 5%. If the particle size of the aggregate is reduced for this purpose, the binder content has to be increased in order to maintain adequate viscosity for application with a trowel at room temperature, but increased amounts of binder result in higher shrinkage rates.
There is accordingly a need for a cathode block cement which is sufficiently fluid for use at ambient temperature and which has a linear shrinkage of less than about 5% when exposed to cell operating conditions as well as having the other necessary characteristics of a cathode block cement, e.g. a suitable carbon yield and density.